Answer Questions

Many times we will ask for more information about the bug or ask you to try various things. Please track your bug report(using bookmarks, subscriptions, or "View your posts" feature) and answer when more details are requested.

In this situation, your bug will most probably be moved to "Assigned" or "Pending", so look for it there.

Factorio is a quite CPU demanding game when building big factories, even on high-end processors the game can reach a point when the factory can no longer be simulated at full speed.

FPS and UPS:
When in game you can press F4 and select "show-fps". This will show you 2 counters: FPS and UPS.UPS: The number of game updates per second. Normally around 60, if it's going below 60 it means that your factory is too big and your CPU can't keep up.FPS: The number of frames per second drawn on the screen. It can never go above the UPS.
If FPS are below UPS, it means the graphics card can't keep up, reducing the graphic settings can compensate that.

Effects of low UPS:
In a singleplayer or multiplayer game with the server being the limiting factor, the game will simply run slower.
On the other hand, if a client can not keep up with the server, he will experience significant lags, freezes and sluggishness because the server will not slow down and so he will constantly fall behind the server, catchup in the background and fall behind again, this is usually unplayable for the client.

A server admin can reduce the simulation speed with the following command to enable the slower players to keep up:/c game.speed=v
with 0<v<1 (0.8 is usually a good compromise), v=1 will restore normal speed.
Keep in mind that with this the game will run slower for all players.

How to report performance problemsYou can report them as a bug if:

You believe your hardware should run the game properly. See system requirements. Remember: High resolution sprites are only supported on high end systems.

or you have found a feature or method of the game that performs particularly bad and can be optimized.

(OBLIGATORY) Screenshot of the game with the following debug options: show-fps, show-time-usage, show-gpu-time-usage, show-multiplayer-waiting-icon, show-multiplayer-statistics. (press F4 and select those options. No additional debug settings may be activated!). In case of occasional FPS/UPS drops, include 2 screenshots: one when there are no performance problems and one when your FPS/UPS drops.

For very strange performance problems that we can't reproduce, an Event Tracing for Windows (ETW) report can help us. You can use this tool to capture a report. If you include this, consider sending the report in private since it can contain sensitive information.

We normally don't investigate modded games. Only report performance bugs for modded games if you believe the slowdown is caused by our mod API or if it's strictly a modding related performance problem.

If the files are too big for a direct forum attachment, post a link to a 3rd party hosting solution e.g.:google-drive, Dropbox, WeTransferThank you!

Background
Our multiplayer code uses Deterministic Lockstep to synchronize clients, this is a method of synchronizing a game from one computer to another by sending only the user inputs that control that game, rather than networking the state of the objects in the game itself.

It means that all players need to simulate every single tick of the game identically. If any computer does something ever-so-slightly different, a desynchronization (desync) occurs. In that case that player will automatically re-download the map from the server in order to get back in sync. Unfortunately making a fully deterministic game is not easy, so you will notice desyncs, especially at the beginning of a new experimental release.

Reporting Desyncs
When a desync occurs, the game generates a desync report. This report contains the game state from the server and the client and by looking at the differences, we can try to determine what went wrong. Please post them as a bug report in the bug report forum.

The desync report: You will find it in the archive folder in the Application directory (%AppData%/Factorio/archive for Windows). There you will find files that look like this: desync-report-2017-04-28_15-46-04.zip. Take the relevant one and upload it to a 3rd party hosting service like google-drive, Dropbox, WeTransfer and share the link.

As much context and details as possible, such as what you and other players were doing as the desync occurred, especially if the desync happens often. Finding a way to reproduce the desync, such as "load this save, wait for friend to connect, rotate this inserter and desync occurs" means we can identify the cause and fix it very quickly.

Important:

In order for the desync-report to be created, it is necessary that after the desync, the desynced client stays connected to the server until the server information is completely downloaded, this takes significantly longer than the normal map download.

Mods, if written incorrectly can cause desyncs very easily and it is not feasible for the factorio devs to fix bugged mods, therefore desync bug reports of modded servers are to be created in a separate subforum. Those reports must additionally include the complete .ziped mods folder.